1. Field of the Invention
The present invention relates to photomasks, and a method for forming a pattern. More specifically, the present invention relates to photomasks used for forming fine patterns on a film to be processed using lithography, and a method for forming patterns using such photomasks.
2. Background Art
In recent years, with the high integration and miniaturization of semiconductor devices, the improvement of resolution in the photolithography has been demanded. In the photolithography, a critical resolution R, which is the critical pattern dimension that can be resolved, is represented by the following equation (1):R=k1·λ/(NA)  (1)where k1 is a constant dependant on image-forming conditions and resist conditions, λ (nm) is the wavelength of exposing light, and NA is the numerical aperture of the taking lens.
Therefore, in order to improve resolution, the wavelength λ of exposing light should be shortened, or the numerical aperture of the lens should be enlarged. However, the pattern size required by exposing techniques has been diminished, and the formation of patterns smaller than the realizable critical resolution R determined by the wavelength λ of exposing light and the numerical aperture of the lens NA has been demanded.
For example, in a semiconductor device having a multi-layer interconnection structure, the formation of a 1:1 line-and-space pattern (hereafter referred to as L/S pattern), which is the same as the gate pitch, is required in the area in the vicinity of the lowermost layer, that is, in the vicinity of the contact hole. For example, since the gate pitch is about 130 nm in the 65-nm technological node, the formation of the L/S pattern having this pitch is required in the vicinity of the lowermost layer of such a semiconductor device. However, it is difficult to cope with such a fine pattern only by shortening the wavelength and the enlarging numerical aperture.
Therefore, in order to form a pattern finer than the critical resolution R of the exposure apparatus, the combination of the technique known as resolution enhancement technique with shortening the wavelength and the enlarging numerical aperture has been considered. There are two techniques of resolution enhancement, for illumination and for the mask.
Specifically, the resolution enhancement for illumination is a technique using an off-axis illumination method. This method improves resolution by adding an aperture under the exposing light source, and for example, the annular illumination, which is one of the techniques of the off-axis illumination methods, by shielding the center portion of the luminous flux to decrease the image components by three-beam interference, and increase two-flux interference components.
On the other hand, a resolution enhancement technique for the mask uses a phase-shifting mask. While a conventional chromium mask controls only the amplitude of light, the phase-shifting mask improves resolution utilizing the phase contrast of light. The examples of phase-shifting masks include an attenuated phase-shifting mask and an alternating (Levenson-type) phase-shifting mask (e.g., refer to Japanese Patent Application Laid-Open No. 7-181668).
The combination of these two resolution enhancement techniques, i.e., off-axis illumination and the phase-shifting mask, is often used for the formation of fine patterns.
The off-axis illumination method depends on the layout of the pattern, considerably. For example, for only lines running in the vertical direction or in the horizontal direction, a dipole illuminating light source is effective for the strong contrast of light. When there are line patterns in both vertical and horizontal directions, a quadrapole illuminating light source is effective. Furthermore, when there are no limitations in the direction or the angle of the lines, annular illumination is effective.
The optimum location of the aperture depends on the pattern pitch of the cycle pattern. Therefore, the off-axis illumination method is particularly effective for cyclic patterns, but the effect is reduced unless the pattern pitch is made constant to some extent. For patterns that are not periodic, for example, a pattern having different pitches, an isolated pattern, or a pattern located in the end of cyclic pattern, the light intensity profiles become significantly different. Therefore, for patterns having no periodicity, resolution or focal depth is made worse rather than being improved as compared to the use of ordinary illumination.
As a measure for correcting dimensional differences in patterns that are not periodic, for example, the use of OPC (optical proximity correction) can be considered. However, although OPC is effective for the correction of dimensions, process margin, such as focal depth and exposure margin, is left unchanged.
FIG. 44 is a schematic diagram for illustrating a photomask whereon a pattern is formed. FIG. 45 is a graph for illustrating the relationship between defocus (μm) and the dimension (nm) of each transferred line pattern, when the pattern of the photomask is transferred.
Here, the photomask is an attenuated phase shifting mask having a transmittance of 5% whereon an L/S pattern of 130-nm pitch is laid out.
Here, since dimensional correction is performed by OPC, the dimensions of lines at best focus are substantially uniform, each other, as FIG. 45 shows. However, in the case of defocus, although the dimensions of Line 2 and Line 3 in the center portion in periodicity pattern are not changed, the dimensions of Line 1 in the end portion and isolated Line 4 are significantly changed to be smaller.
When the conventional off-axis illumination method or phase shifting masks are used, the process margin for the pattern out of periodicity is low, and even if dimensional correction by OPC is used, there is limitation in the accurate transfer of patterns. Therefore, in the pattern portion whereon a pattern out of periodicity is transferred, difference from the designed pattern dimension increases, and therefore, defects such as short-circuiting and disconnection often occur in this area, causing problems.